Process for preparing paclitaxel

ABSTRACT

The present invention elates to a process for preparing paclitaxel represented by formula (1) characterized in that: (a) an oxazolidine derivative represented by formula (2) or its salt in which X represents halogen, is coupled with a 7-trihaloacetyl-baccatin III represented by formula (3) in which R 1  represents trihaloacetyl, in a solvent in the presence of a condensing agent to produce an oxazolidine substituent-containing taxane represented by formula (4) in which X and R 1  are each as previously defined; (b) the oxazolidine ring is opened in a solvent in the presence of an acid, and the product thus obtained is reacted with benzoyl chloride in the presence of a base to produce a protected paclitaxel wherein the hydroxy group at 7-position is protected with trihaloacetyl group represented by formula (5) in which R 1  is as previously defined; (c) then the protecting group at 7-position is removed by ammonia or a salt of ammonia with a weak acid in a solvent.

TECHNICAL FIELD

The present invention relates to a novel process for preparingpaclitaxel represented by the following formula (I): ##STR2## in whichPh represents phenyl,

Ac represents acetyl,

Bz represents benzoyl, and

hereinafter they have the same meaning.

The present invention also relates to novel intermediates which may beused in the process for preparing paclitaxel of formula 1; and toprocesses for preparing them.

BACKGROUND ART

Paclitaxel of formula1, a terpene taxane derivative, is a potentanti-tumor chemotherapeutics having a broad spectrum of anti-leukemiaand anti-tumor activity. Accordingly, many concerns have been focused onthis compound in both area of biology and chemistry. Paclitaxel has alsobeen allowed to be commercially marketed as an anti-tumor agent againstovarian cancer and breast cancer in several countries including theUnited States.

Hitherto, paclitaxel has been provided by separation from the bark ofTaxus brevifolia which is a kind of western yew tree. However, since theseparation and purification procedures are very burdensome and furtheronly small amount of paclitaxel is contained in the bark of thatevergreen tree, the amount of paclitaxel thus provided can hardly meetthe more and more increasing commercial need.

Recently, the chemists have extensively studied about semi-syntheseswhich are applicable for preparing paclitaxel and about new syntheticmethods for the same compound including processes for preparing theintermediates. However, a lot of synthetic methods reported heretoforehave not shown a satisfactory result.

For example, WO 93/06094 discloses a process for preparing paclitaxel byreacting a beta-lactam compound and 7-triethylsilyl-baccatin III, thenby deprotecting, as depicted in the following reaction scheme 1. Now,this is recognized as the shortest procedure to obtain the desiredpaclitaxel compound. ##STR3## in which TES represents triethylsilyl, andhereinafter has the same meaning.

Although this method has a merit that the coupling reaction can bereadily carried out by using the beta-lactam compound, it also has manydisadvantages such that the synthesis of the beta-lactam compound itselfis very difficult, the coupling reaction should be proceeded at a lowtemperature of -45° C. under an anhydrous condition, a toxic acid havinga strong corrosive action against glass products as well as propertieshard to treat (i.e.,48% HF) should be used during the process forremoving the triethylsilyl (TES) group used as a protecting group for ataxane derivative, etc.

In addition, as depicted in the following reaction scheme 2, a processwherein an oxazolidine compound instead of the beta-lactam is coupledwith a 7-Troc-baccatin III in the presence of dicyclohexylcarbodiimideor 2-dipyridylcarbonate is described in Commercon et al., TetrahedronLetters, pp5185-5188, 1992. ##STR4## in which Boc representst-butoxycarbonyl,

Troc represents trichloroethoxycarbonyl, and hereinafter they have thesame meaning.

In the above process for preparing paclitaxel, a new protecting groupfor the taxane derivative (-Troc) which is different from that used inthe prior process of reaction scheme 1 (-TES) has been used. However,according to the use of this novel protecting group, more reaction stepsshould be carried out to obtain the desired product, vigorous reactionconditions are required for the protection as well as deprotectionreaction, and as a result the total yield becomes low. Therefore, theprocess of reaction scheme 2 is understood inferior to that of reactionscheme 1.

In the process of reaction scheme 2, an oxazolidine derivative iscoupled with a taxane derivative to prepare paclitaxel. As the knownoxazolidine derivatives which have been used for such a purpose, acompound represented by the following formula 6 (see, WO 94/10169) and acompound represented by the following formula 7 (see, Korean PatentAppln. No. 95-703548) can be mentioned. ##STR5## in which R_(a)represents phenyl or R_(f) O, wherein R_(f) represents alkyl, alkenyl,alkynyl, cycloalkyl, cycloalkenyl, phenyl or heterocyclic compoundcontaining nitrogen atom,

R_(b) and R_(c) independently of one another represent hydrogen, alkyl,phenylalkyl, phenyl, alkoxyphenyl or dialkoxyphenyl, or

R_(b) and R_(c) can form a cyclic chain having 4 to 7 ring atoms.##STR6## in which R_(d) represents hydrogen, benzoyl or R_(g) OCO--,wherein R_(g) represents alkyl, alkenyl, cycloalkyl, cycloalkenyl orbicycloalkyl, R_(e) represents trihalomethyl or phenyl substituted bytrihalomethyl.

The oxazolidine derivative of formula 6 above has various substituentsat 2-position of the ring and the amine group at 3-position isnecessarily protected by a substituted carbonyl group. It is because theamine group at 3-position is apt to react with the carboxylic acid at5-position of another oxazolidine molecule if it is not protected. Thiskind of side reaction has greater reactivity than the coupling reactionbetween the oxazolidine derivative and the taxane derivative, and as aresult the coupling reaction cannot be carried out favorably. Anothercase can be seen from the oxazolidine derivative of formula 7. That is,in case the substituent at 2-position of the oxazolidine ring istrihalomethyl or phenyl substituted by trihalomethyl, the desiredcoupling reaction can be carried out without any side reaction such asself coupling between oxazolidine molecules although the amine group at3-position is not protected. If no protection for the amine group at3-position of oxazolidine ring is required, several advantages includingno need for removing the protecting group at a later step may beanticipated.

While, another process for preparing paclitaxel, as depicted in thefollowing reaction scheme 3, is disclosed in Korean Patent Appln. No.94-702930. ##STR7##

In the above process, paclitaxel is prepared by coupling an oxazolinederivative with a taxane derivative having a hydroxy group directlyattached to C-13 to produce a taxane derivative having an oxazolinesubstituent, opening the oxazoline ring, then deprotecting. When such anoxazoline derivative substituted by a phenyl group at 2-position and ataxane derivative protected with triethylsilyl group are used asstarting materials, however, opening reaction of the oxazoline ring anddeprotection reaction may not readily performed. That is, severereaction conditions such as strong acidity and long reaction time (i.e.,IN-HCl (5.5 eq.), 0° C., 21 hrs) are required, nevertheless,productivity and reaction yield are still low.

As aforementioned, the earlier developed processes for preparingpaclitaxel use various side chain substituents such as beta-lactamderivative, oxazolidine derivative, or oxazoline derivative. But, allthose substituents have some problems, for example, synthesis of itselfor coupling with a taxane derivative is difficult, opening thesubstituent ring requires severe conditions, etc.

Another important point that should be considered is the choice of theprotecting group for taxane derivative. As the protecting groups fortaxane derivative reported up to the present, triethylsilyl andtrichloro-ethoxycarbonyl can be mentioned. In order to remove suchprotecting groups, extremely acidic conditions or extended reaction timesuch as reacting for 14 hours in the presence of 48HF/pyridine; reactingfor 4 hours in formic acid solvent; reacting at a condition of zinc,AcOH/MeOH=1/1 and 60° C.; reacting for 30 hours in 0.5% HCl/EtOH; etc.are usually required. Moreover, those protecting groups have thefollowing problems. In the case of triethylsilyl group, an excessiveamount (20 eq.) of the expensive triethylsilyl chloride should be usedin the protection reaction (see, WO 93/06094); and in the case oftrichloroethoxycarbonyl group, complicated synthetic pathway should beapplied for preparing the protected taxane derivative since thisprotecting group has no selectivity (see, Commercon, et al., TetrahedronLetters, pp5185-5188, 1992).

DISCLOSURE OF INVENTION

Thus, the present inventors have extensively studied for years todevelop a more improved process for preparing paclitaxel by solving thevarious problems of prior art processes as mentioned above. As a result,we have identified that the opening reaction of the side chainsubstituent and deprotection reaction can be easily performed under verymild conditions if coupling is carried out between a novel oxazolidinederivative wherein two halomethyl substituents are present at 2-positionand the amine at 3-position is not protected and a novel taxanederivative wherein the hydroxy group at 7-position is protected bytrihaloacetyl. Particularly, the present inventors have found asurprising fact that if the novel oxazolidine derivative having twohalomethyl substituents at 2-position is used, the coupling reaction ofthe oxazolidine derivative with the taxane derivative may well becarried out without any side reaction such as self coupling betweenoxazolidine molecules although the amine at 3-position is not protected.

Therefore, it is an object of the present invention to provide a novelprocess for preparing paclitaxel represented by the following formula 1##STR8## characterized in that (a) an oxazolidine derivative representedby the following formula 2 or its salt: ##STR9## in which X representshalogen, is coupled with a 7-trihaloacetyl-baccatin III represented bythe following formula 3: ##STR10## in which R₁ represents trihaloacetyl,in a solvent in the presence of a condensing agent to produce anoxazolidine substituent-containing taxane represented by the followingformula 4: ##STR11## in which X and R₁ are each as previously described;(b) the oxazolidinie ring is opened in a solvent in the presence of anacid, and the product thus obtained is reacted with benzoyl chloride inthe presence of a base to produce a protected paclitaxel wherein thehydroxy group at 7-position is protected with trihaloacetyl grouprepresented by the following formula 5: ##STR12## in which R₁ is aspreviously described; (c) then the protecting group at 7-position isremoved by ammonia or a salt of ammonia with a weak acid in a solvent.

It is another object of the present invention to provide novel startingmaterials oxazolidine derivative of formula 2 and taxane derivative offormula 3, and processes for preparing them.

BEST MODE FOR CARRYING OUT THE INVENTION

The process for preparing paclitaxel according to the present inventioncan be summarized as the following reaction scheme 4 ##STR13## in whichX and R.sub. are each as previously described.

The process described in the above reaction scheme 4 will be morespecifically explained below.

The step (a) reaction for producing the oxazolidinesubstituent-containing taxane of formula 4 by coupling the7-trihaloacetyl-baccatin III of formula 3 with the oxazolidinederivative of formula 2 is preferably carried out at temperaturesranging from 20 to 60° C. Solvents suitable for this reaction includeethers such as tetrahydrofuran, diisopropylether, methyl t-butylether ordioxane; ketones such as methyl isobutyl ketone; nitriles such asacetonitrile; esters such as ethylacetate, isopropylacetate orn-butylacetate; aliphatic hydrocarbons such as pentane, hexane orheptane; chlorinated hydrocarbons such as dichloromethane, chloroform or1,2-dichloroethane; aromatic hydrocarbons such as benzene, toluene orxylene; amides such as dimethylacetamide or dimethylformamide. In thecoupling reaction, the oxazolidine derivative of formula 2 is used in anexcessive amount, for example, an equimolar amount to 3 times molaramount, preferably 1.5 to 3 times molar amount, with respect to thecompound of formula 3. This coupling reaction is carried out in thepresence of a condensing agent, and optionally in the presence of anactivating agent. As the condensing agent, carbodiimides such asdicyclohexylcarbodiimide, and reactive carbonates such as2-dipyridylcarbonate can be mentioned. And as the activating agent,dialkylaminopyridines such as 4-dimethylaminopyridine,4-pyrrolidinopyridine and the like can be mentioned. Generally, thecondensing agent is used in a stoichiometric amount with respect to theoxazolidine compound of formula 2, and the activating agent is used in astoichiometric amount or less with respect to the7-trihaloacetyl-baccatin III of formula 3.

The step (b) reaction for preparing the compound of formula 5 comprisesopening the oxazolidine ring and reacting the opened product withbenzoyl chloride in the presence of a base. This reaction merelyrequires a weak acidic condition wherein 1 to 1.5 equivalents of acidwith respect to the compound of formula 4 is used, which is an effectresulted from the use of the novel oxazolidine derivative of formula 2as a starting material. Acids which can be used for adjusting theacidity of the reaction solution include hydrochloric acid, sulfuricacid, formic acid, nitric acid, acetic acid, trifluoroacetic acid,p-toluenesulfonic acid, methanesulfonic acid and benzoic acid. The acidcan be used in a stoichiometric amount with respect to the compound offormula 4, however, 1.5 equivalents of acid is preferably used in orderto complete the reaction within the shortest time and to minimize theside reactions. As the solvent, one or more selected from a groupconsisting of ethers such as tetrahydrofuran, diethylether or dioxane;nitriles such as acetonitrile, ketones such as acetone or methylisobutyl ketone, esters such as ethylacetate, isopropylacetate orn-butylacetate; chlorinated hydrocarbons such as dichloromethane,chloroform or 1,2-dichloroethane; aromatic hydrocarbons such as benzene,toluene or xylene; and amides such as dimethylacetamide ordimethylformamide can be used. This reaction is preferably carried outat temperatures ranging from -20 to 60° C. for 10 to 30 minutes.

The reaction solution thus obtained is neutralized with a suitable base,diluted with water and then reacted with benzoylchloride to produce thepaclitaxel protected with trihaloacetyl at 7-position of formula 5. Thereactant benzoylchloride is used in a stoichiometric amount, morespecifically 1 to 1.2 equivalents, with respect to the oxazolidinesubstituent-containing taxane of formula 4. As the base, one or morewater-soluble bases selected from a group consisting of sodiumbicarbonate, potassium bicarbonate, sodium carbonate, potassiumcarbonate, sodium hydroxide, potassium hydroxide and lithium hydroxidemay be used in an excessive amount, preferably 3 to 20 equivalents, withrespect to the compound of formula 4.

Finally, in step (c), the trihaloacetyl group at 7-position of thecompound 5 is removed by using ammonia or a salt of ammonia with a weakacid to prepare the desired compound paclitaxel of formula 1. In thisreaction, aqueous ammonia or ammonia-organic solvent solution having aconcentration of 5 to 40% may be used in a stoichiometric amount ormore, preferably 1 to 5 equivalents, with respect to the compound offormula 5. When a salt of ammonia with a weak acid replaces ammonia, itis used in an amount of 1 to 5 equivalents with respect to the compoundof formula 5. The weak acid can be formic acid, acetic acid or propionicacid. As the solvent, one or more selected from a group consisting ofalcohols such as methanol, ethanol or isopropyl alcohol; ethers such astetrahydrofuran, diethylether or dioxane; nitriles such as acetonitrile;esters such as ethylacetate, isopropylacetate or n-butylacetate;chlorinated hydrocarbons such as dichloromethane, chloroform or1,2-dichloroethane; aromatic hydrocarbons such as benzene, toluene orxylene; and amides such as dimethylacetamide or dimethylformamide can beused. This reaction is preferably carried out at temperatures rangingfrom 0 to 60° C. Hitherto, very strong acidic conditions have generallybeen required to eliminate the protecting group at 7-position of taxane.However, the present invention makes it possible to readily remove theprotecting group under weak alkali or almost neutral conditions whereinammonia or a salt of ammonia with a weak acid is used.

The 7-trihaloacetyl-baccatin III of formula 3 used as a startingmaterial in the process for preparing paclitaxel is itself a novelcompound, therefore it is another object of the present invention toprovide the novel taxane derivative of formula 3.

The 7-trihaloacetyl-baccatin III of formula 3 can be prepared by (d)reacting a 10-deacetyl-baccatin III represented by the following formula8: ##STR14## with trihaloacetyl halide in a solvent in the presence of abase to provide a 10-deacetyl-7-trihaloacetyl-baccatin III representedby the following formula 9: ##STR15## in which R₁ representstrihaloacetyl, then (e) reacting the compound of formula 9 thus obtainedwith acetyl halide in a solvent in the presence of a base.

The process for preparing the compound of formula 3 can be summarized asthe following reaction scheme 5. ##STR16## in which R₁ is as previouslydescribed.

In step (d) of the reaction scheme 5, the hydroxy group at 7-position ofthe 1 0-deacetyl-baccatin III of formula 8 is protected by trihaloacetylto provide the 10-deacetyl-7-trihaloacetyl-baccatin III of formula 9. Inthe earlier processes using triethylsilyl or trichloroethoxycarbonyl asa protecting group, about 20 times molar excess of triethylsilyl shouldbe used or some annoying procedure consisting of several steps should becarried out. In contrast, this step according to the present inventionuse only a stoichiometric amount of trihaloacetyl to selectively protectthe hydroxy group at 7-position of the taxane derivative. One selectedfrom a group consisting of trichloroacetyl chloride, trichloroacetylbromide, tribromoacetyl chloride, tribromoacetyl bromide,trifluoroacetyl chloride, trifluoroacetyl bromide, triiodoacetylchloride and triiodoacetyl bromide can be used as the trihaloacetylhalide, and it is usually used in a stoichiometric amount or more,preferably 1 to 1.5 equivalents, with respect to the compound of formula8. Solvents which can be suitably used for this reaction include etherssuch as tetrahydrofuran, diisopropylether, methyl t-butylether ordioxane; ketones such as methyl isobutyl ketone; nitrites such asacetonitrile; esters such as ethylacetate, isopropylacetate orn-butylacetate; aliphatic hydrocarbons such as pentane, hexane orheptane; chlorinated hydrocarbons such as dichloromethane, chloroform or1,2-dichloroethane; aromatic hydrocarbons such as benzene, toluene orxylene; amides such as dimethylacetamide or dimethylformamide; and basicsolvents such as pyridine. When pyridine is used as the solvent, thereaction can be performed without a base. As the appropriate base,pyridine, triethylamine, imidazole, DBU, diisopropylethylamine,potassium t-butoxide, sodium ethoxide, n-butyllithium, phenyllithium,lithium diisopropylamide, sodium hydride or lithiumbistrimethylsilylamide can be mentioned This reaction is preferablycarried out at temperatures ranging from -20 to 60° C.

In step (e), the novel 7-trihaloacetyl-baccatin III of formula 3 isprepared by reacting the 1-deacetyl-7-trihaloacetyl-baccatin III offormula 9 with acetyl halide in a solvent in the presence of a base. Theacetyl halide is used in a stoichiometric amount or more, preferably 1to 8 equivalents, with respect to the10-deacetyl-7-trihaloacetyl-baccatin III of formula 9. Solvents andbases which can be suitably used, and reaction temperature range are thesame as mentioned for step (d).

An oxazolidine derivative represented by the following formula 2a isitself a novel compound, therefore it is yet another object of thepresent invention. ##STR17## in which X is as previously described, and

R represents hydrogen or C₁ -C₃ alkyl.

The most important feature of this compound is that two halomethylsubstituents are present at 2-position of the oxazolidine ring and theamine at 3-position is not protected. In addition, this compoundincludes the compound of formula 2 which is the starting material of theprocess for preparing paclitaxel according to the present invention.

The oxazolidine derivative of formula 2a as defined above can beprepared by reacting a (2R,3S)-phenylisoserine derivative represented bythe following formula 10: ##STR18## in which R is as previouslydescribed, with a compound represented by the following formula 11:##STR19## in which X is as previously described, in a solvent in thepresence of an acid catalyst.

The compound of formula 2a which is prepared after carrying out theabove process includes two types of compounds, one is a free acid form(R=hydrogen) and the other is an ester form (R=alkyl). Therefore, if thefree acid form is desirable, hydrolysis may be further carried out onthe ester form compound of formula 2a.

All the earlier processes for preparing the oxazolidine derivative haveused only the phenylisoserine alkylester (ester form) as the startingmaterial.

However, in the present invention, the oxazolidine derivative can beprepared from phenylisoserine as well as phenylisoserine alkylester.That is, the present inventors have found an astonishing fact that thenovel oxazolidine derivative of formula 2 (R=hydrogen) can be directlyprepared from phenylisoserine. In case phenylisoserine, not ester form,is used as the starting material, two steps (one step for preparingphenylisoserine methylester from phenylisoserine and the other step forremoving the methylester group from the oxazolidine derivative) can besaved, therefore it is another advantage resulted from the presentinvention.

The process for preparing the oxazolidine derivative of formula 2a canbe depicted as the following reaction scheme 6: ##STR20## in which X andR are each as previously described.

The oxazolidine derivative of formula 2a wherein R is hydrogen or alkylis prepared by reacting the (2R,3S)-phenylisoserine derivative with thehalogenated acetone of formula 11 in a solvent in the presence of anacid catalyst. As the acid catalyst p-toluenesulfonic acid,methanesulfonic acid, trifluoromethylsulfonic acid, pyridiniump-toluenesulfonate, amberlite IR-120, etc. can be used, and as thesolvent one or more selected from a group consisting of ethers such astetrahydrofuran, diisopropylether, methyl t-butylether or dioxane;nitriles such as acetonitrile; esters such as ethylacetate,isopropylacetate or n-butylacetate; chlorinated hydrocarbons such asdichloromethane, chloroform or 1,2-dichloroethane; aromatic hydrocarbonssuch as benzene, toluene or xylene; and amides such as dimethylacetamideor dimethylformamide can be used. When the (2R,3S)-phenylisoserine(R=hydrogen), which is hard to solve in solvent used, is used as thestarting material, a good yield may be obtained by adding a smallquantity of amides into the reaction solution, and also the curtailmentof reaction time can be accomplished by adding the amides in an amountof 1/20 times or more with respect to the total volume of solvent. Thisreaction is preferably carried out at temperatures ranging from 30 to100° C.

When the compound of formula 2a wherein R is alkyl is obtained, it canbe further hydrolyzed to the compound of formula 2a wherein R ishydrogen. Water-soluble bases such as hydroxides can be used as thehydrolyzing agent. The hydroxides which can be used for this purposeinclude alkali metal hydroxides such as, for example, lithium hydroxide,sodium hydroxide, potassium hydroxide, etc. The hydrolyzing agent ispreferably used in an amount of 1 to 2 equivalents with respect to theoxazolidine compound of formula 2a wherein R is alkyl. As the reactionsolvent, a solvent mixture of water-miscible organic solvent and waterin a ratio of 10:1 to 100:1 by volume can be used. Water-miscibleorganic solvents which can be suitably used herein include methanol,ethanol, isopropyl alcohol, tetrahydrofuran, dimethylformamide,dimethylacetamide, acetone and acetonitrile. This reaction is preferablycarried out at temperatures ranging from -20 to 60° C. After thehydrolysis is completed, the organic solvent is removed by distillationunder reduced pressure to obtain the oxazolidine compound of formula 2awherein R is hydrogen in the form of an inorganic salt (RCO₂ M). Inorder to convert the salt form into a free acid form (RCO₂ H), an acidor buffer solution having pH 5 to 7 is used. Acids which can be used forthis purpose include hydrochloric acid, formic acid, acetic acid andtrifluoroacetic acid. The acid is used in the same equivalent to thealkali metal hydroxide used in the hydrolysis.

Since the ring opening reaction after coupling may be readily carriedout under mild conditions when the compound of formula 2a is used, thiscompound according to the present invention is more advantageous thanthe existing 2-phenyl-2-oxazoline derivative which requires a verystrong acidic condition and long reaction time (that is, 1N-HCl (5.5eq.), 0° C., 21 hrs) during the ring opening and deprotection reaction.

The (2R,3S)-phenylisoserine derivative or its salt which are used as astarting material in the reaction of scheme 6 can be obtained by aprocess known in the art (see, U.S. Pat. No. 5,420,337; Commercon etal., Tetrahedron Letters, 33, pp5185-5188, 1992; and Kim et al., KoreanPatent Appln. No. 96-7304).

Since the oxazolidine compound of formula 2 (R=hydrogen) thus obtainedis unstable in the presence of an acid, it is desirable to store thecompound in a salt form with a tertiary amine base as represented by thefollowing formula 2b: ##STR21## in which X is as previously described,

R₅, R₆ and R₇ independently of one another represent C₁ -C₄ alkyl orphenylalkyl, or

R₅ and R₆ together can form a cyclic chain having 4 to 7 ring atoms

The present invention will be more specifically explained by thefollowing examples. However, it should be understood that the examplesare intended to illustrate but not to in any manner limit the scope ofthe present invention.

EXAMPLE 1 Synthesis of 10-deacetyl-7-trichloroacetyl-baccatin III

30 g (0.055 mol) of 10-Deacetyl-baccatin III was dissolved in 1.35 l ofchloroform, 134 ml (30 eq.) of pyridine was added dropwise thereto. andthen the reaction mixture was stirred for 10 minutes. After 12 g (0.066mol) of trichloroacetyl chloride was slowly added, the resulting mixturewas stirred at room temperature for 30 minutes. Then, 3 g (0.017 mol) oftrichloroacetyl chloride was further added thereto and the whole mixturewas stirred for 20 minutes. The solvent was removed under reducedpressure, 100 ml of water was added to the residue, and then theresulting solution was extracted with 700 ml of ethylacetate. Theorganic layer thus obtained was washed with aqueous sodium chloridesolution, dried over anhydrous magnesium sulfate, and then distilledunder reduced pressure. The residue was dissolved in 200 ml of tolueneand then cooled down to 0° C. The resulting fine solid was filtered andthen washed with 200 ml of hexane to obtain 36 g (Yield 95% ) of thepure title compound.

M.P.: 216° C.; [a]_(D) ²⁵ =-45.3° (c=1, CHCl₃); ¹ H NMR (300 MHz,CDCl₃):δ 8.12(d,J=7.3 Hz,2H), 7.67-7.49(m,3H), 5.67(d,J=6.9 Hz,1H),5.61-5.55(m,1H), 5.35(s,1H), 5.00(d,J=8.6 Hz,1H), 4.90(m,1H),4.38(d,J=8.5 Hz,1H), 4.22(d,J=8.4Hz,1H), 3.99(m,1H), 2.77-2.71(m,1H),2.32-l.09(m,18H).

EXAMPLE 2 Synthesis of 7-trichloroacetyl-baccatin III

36 g (0.052 mol) of 10-deacetyl-7-trichloroacetyl-baccatin III preparedin Example 1 was dissolved in 1.25 l of chloroform. 125 ml (30 eq.) ofpyridine was added dropwise thereto and the mixture was stirred for 10minutes. To the mixture was slowly added 38 g (0.309 mol) of acetylbromide, then the resulting mixture was stirred at room temperature for3 hours. The solvent was removed under reduced pressure, 100 ml of waterwas added to the residue, and then the resulting solution was extractedwith 700 ml of ethylacetate. The organic layer thus obtained was washedwith aqueous sodium chloride solution, dried over anhydrous magnesiumsulfate, and then distilled under reduced pressure. The residue wasdissolved in 200 ml of toluene and then cooled down to 0° C. Theresulting fine solid was filtered and then washed with 200 ml of hexaneto obtain 35.9 g (Yield 94% ) of the pure title compound.

M.P.: 180° C.; [a]_(D) ²⁵ =-62.3 (c=1, CHCl₃); ¹ H NMR(300 MHz,CDCl₃): δ8.13(d,J=7.3 Hz,2H), 7.67-7.49(m,3H), 6.50 (s,1H), 5.76-5.67(m,2H),5.00(d,J=8.8 Hz,1H), 4.89(m,1H), 4.37(d,J=9.0 Hz,1H), 4.19 (d,J=9.4Hz,1H), 4.06(m,2H), 2,73-2.69(m,1H), 2.34-1.11(m,21H).

EXAMPLE 3 Synthesis of (4S;5R)-2,2'-di(chloromethyl)-4-phenyl-1,3-oxazolidine-5-carboxylic acid methylester

10.0 g (0.051 mol) of (2R,3S)-phenylisoserine methylester, 6.51 g (0.051mol) of 1,3-dichloroacetone and 0.1 g of pyridinium p-toluenesulfonatewere dissolved in 100 ml of acetonitrile. The mixture was stirred at 80°C. for 40 minutes and then the reaction solvent was removed bydistillation under reduced pressure. 100 ml of water was added to theresidue, which was then extracted with 200 ml of ethylacetate. Themoisture contained in the organic layer was eliminated over anhydrousmagnesium sulfate and then the solvent was distilled off under reducedpressure to obtain 14.1 g (Yield 90% ) of the title compound.

¹ H NMR (300 MHz,CDCl₃): δ 7.47-7.28 (m,5 H), 4.64-4.53(m,2 H), 3.95(s,2H), 3.84(d,J=6.0 Hz,2 H), 3.78(s,3 H), 3.10(d,J=10.2 Hz,1 H).

EXAMPLE 4 Synthesis of(4S,5R)-2,2'-di(chloromethyl)-4-phenyl-1,3-oxazolidine-5-carboxylic acidtriethylamine salt (Method I)

After 13.0 g (0.043 mol) of (4S,5R)-2;2'-di(chloromethyl)-4-phenyl-1,3-oxazolidine-5-carboxylic acid methylester prepared in Example 3 wasdissolved in 50 ml of methanol, 15.7 ml (0.047 mol) of 3.0N aqueouslithium hydroxide solution was added dropwise thereto. The reactionmixture was stirred at normal temperature for 30 minutes and thenreaction solvent was removed by distillation under reduced pressure. 30ml of water and 15.7 ml (0.047 mol) of 3.0N aqueous hydrochloric acidsolution were added to the residue, which was then extracted with 200 mgof ethylacetate. The moisture contained in the organic layer waseliminated over anhydrous magnesium sulfate and then 9.0 ml (0.065 mol)of triethylamine was added dropwise thereto. The solvent was distilledoff under reduced pressure to obtain 15.7 g (Yield 94% ) of the titlecompound.

¹ H NMR (300 MHz,CDCl₃): δ 7.52-7.28 (m,5 H), 4.57(d,J=7.6 Hz,1 H),4.46(d,J=7.6 Hz,1 H), 3.94-3.81(m,4 H), 3.07(q,J=7.1 Hz,6 H),1.28(t,J=7.1 Hz,9 H).

EXAMPLE 5 Synthesis of (4S,5R)-2,2'-di(chloromethyl)-4-phenyl-1,3-oxazolidine-5-carboxylic acid triethylamine salt (Method II)

10.0 g (0.055 mol) of (2R,3S)-phenylisoserine, 7.0 g (0.055 mol) of1,3-dichloroacetone, 5 ml of dimethylformamide, and 0.1 g of pyridiniump-toluenesulfonate were dissolved in 100 ml of acetonitrile. The mixturewas stirred at 80° C. for one hour and then the reaction solvent wasremoved by distillation under reduced pressure. 100 ml of water wasadded to the residue, which was then extracted with 200 ml ofethylacetate. The moisture contained in the organic layer was eliminatedover anhydrous magnesium sulfate and then 11.5 ml (0.083 mol) oftriethylamine was added dropwise thereto. The solvent was distilled offunder reduced pressure to obtain 19 g (Yield 88% ) of the titlecompound.

EXAMPLE 6 Synthesis of13-[(4S,5R)-2,2'-di(chloromethyl)-4-phenyl-1,3-oxazolidinyl-carbonyl]-baccatin III

15.7 g (0.040 mol) of(4S,5R)-2,2'-di(chloromethyl)-4-phenyl-1,3-oxazolidine-5-carboxylic acidtriethylamine salt prepared in Example 4 or 5 and 9.8 g (0.013 mol) of7-trichloroacetyl-baccatin III prepared in Example 2 were dissolved in120 ml of toluene together with 8.2 g (0.040 mol) ofdicyclohexylcarbodiimide and 0.1 g of 4-dimethylaminopyridine. Theresulting mixture was stirred at normal temperature for one hour,filtered through a cellite pad, and then distilled under reducedpressure to remove the reaction solvent. The residue was subjected tocolumn chromatography (eluent: ethylacetate/hexane=1/2, v/v) to obtain12.6 g (Yield 94.5% ) of the title compound as a pale yellow solid.

M.P.: 187° C.; [a]_(D) ²⁵ =-24.8° (c=0.44, CHCl₃); ¹ H NMR(300MHz,CDCl₃): δ 8.03(d,J=7.2 Hz,2 H), 7.68-7.45(m,8 H), 6.39 (s,1 H), 6.26(t,J=8.5 Hz,1 H),5.76-5.69(m,2 H), 5.67-5.62(m,2 H), 4.89 (d,J=8.4 Hz,1H), 4.65(m,1 H), 4.43(d,J=8.2 Hz,1 H), 4.27(d,J=8.4 Hz,1 H),4.11(d,J=7.3 Hz,1 H), 4.04(s,2 H), 3.90(m,1 H), 3.82(d,J=9.8 Hz,2 H),3.20(d,J=10.5 Hz), 2.65(m,1 H), 2.15-1.14(m,21 H).

EXAMPLE 7 Synthesis of Paclitaxel

12 g (0.012 mol) of the coupling product prepared in Example 6 wasdissolved in 50 ml of ethylacetate. 1.5 ml (0.018 mol) of conc.hydrochloric acid was added dropwise thereto and the mixture was stirredat normal temperature for 20 minutes. The reaction solution wasneutralized with 12 g (0.14 mol) of sodium bicarbonate. 100 ml of waterwas added to the mixture and then 1.7 ml (0.014 mol) of benzoyl chloridewas added dropwise thereto. The reaction solution was stirred at normaltemperature for 10 minutes, extracted with 300 ml of ethylacetate, andthen dried over anhydrous magnesium sulfate. The solvent was removed bydistillation under reduced pressure and the resulting residue wasdissolved in 50 ml of a solvent mixture of methanol and tetrahydrofuran(3/1, v/v). 2.0 M Ammonia (6.0 ml, 0.012 mol) dissolved in methanol wasadded dropwise thereto, the resulting mixture was stirred at normaltemperature for one hour, and then the reaction solvent was distilledoff under reduced pressure. The residue was subjected to columnchromatography (eluent: ethylacetate/hexane=2/1, v/v) to obtain 9.39 g(Yield 92% ) of the title compound as a white solid.

¹ H NMR(300 MHz,CDCl₃): δ 8.15(d,J=7.2 Hz,2 H), 7.76(d,J=7.2 Hz,2 H),7.64-7.37(m,11 H), 7.04(d,J=8.8 Hz,1 H), 6.29(s,1 H), 6.25(dd,J=8.9 Hz,J=8.9 Hz,1 H), 5.80(dd,J1=2.5 Hz,J2=8.7 Hz,1 H), 5.69(d,J=7.1 Hz, 1 H),4.95(dd,J 1=2.3 Hz,J2=10 Hz,1 H), 4.81(dd,J 1=2.5 Hz,J2=5.0 Hz, 1 H),(m,1 H), 4.32(d,J=8.5 Hz,1 H), 4.22(d, J=8.5 Hz,1 H), 3.83(d,J=6.9 Hz,1H), 3.66(d,J=5.3 Hz,1 H), 2,54-1.16(m,22 H).

The advantages resulted from the present invention can be summarized asfollows.

The existing processes for preparing paclitaxel generally usetriethylsilyl or trichloroethoxycarbonyl group for the protection of thehydroxy group at 7-position of baccatin III. The removal of theseprotecting groups requires very vigorous condition using strong acid. Inthe present invention, however, the hydroxy group at the same positionis protected by trihaloacetyl group which can be easily removed underweak alkali or almost neutral mild conditions (for example, 1 equivalentof ammonia/organic solvent, 20° C., 1 hr; or 1.2 equivalent of ammoniumacetate/organic solvent, 20° C., 3 hrs). Therefore, according to thepresent invention, paclitaxel can be prepared with facility and moreeconomically. Particularly, the mildness of reaction condition isexpected to be a conspicuous advantage when the process is applied on anindustrial scale.

Another advantage of this invention lies in the use of the noveloxazolidine derivative wherein two halomethyl substituents are presentat 2-position and the amine at 3-position is not protected as asubstituent for baccatin III. This novel oxazolidine derivative does notcause side reactions such as self coupling and makes the ring openingreaction after coupling easy, and thus increases the productivity.Moreover, since this novel oxazolidine derivative can be directlyprepared from phenylisoserine, the curtailment of operation can beaccomplished.

Consequently, by using the appropriate protecting groups which are easyto remove, the present process may be conveniently carried out undermild conditions with a high total yield.

What is claimed is:
 1. A process for preparing paclitaxel represented bythe following formula 1: ##STR22## characterized in that (a) anoxazolidine derivative represented by the following formula 2 or itssalt: ##STR23## in which X represents halogen, is coupled with a7-trihaloacetyl-baccatin III represented by the following formula 3:##STR24## in which R₁ represents trihaloacetyl, in a solvent in thepresence of a condensing agent to produce an oxazolidinesubstituent-containing taxane represented by the following formula 4:##STR25## in which X and R₁ are each as previously defined; (b) theoxazolidine ring is opened in a solvent in the presence of an acid, andthe product thus obtained is reacted with benzoyl chloride in thepresence of a base to produce a protected paclitaxel wherein the hydroxygroup at 7-position is protected with trihaloacetyl group represented bythe following formula 5: ##STR26## in which R₁ is as previously defined;(c) then the protecting group at 7-position is removed by ammonia or asalt of ammonia with a weak acid in a solvent.
 2. The process of claim1, wherein X is chloro and R₁ is trichloroacetyl.
 3. The process ofclaim 1, wherein the solvent of step (a) is one or more selected from agroup consisting of tetrahydrofuran, diisopropylether, methylt-butylether, dioxane, methyl isobutyl ketone, acetonitrile,ethylacetate, isopropylacetate, n-butylacetate, pentane, hexane,heptane, dichloromethane, chloroform, 1,2-dichloroethane, benzene,toluene, xylene, dimethylacetamide and dimethylformamide.
 4. The processof claim 1, wherein the condensing agent of step (a) is carbodiimides orreactive carbonates.
 5. The process of claim 1, wherein the reaction ofstep (a) is carried out in the presence of 4-dimethylaminopyridine or4-pyrrolidinopyridine as an activating agent.
 6. The process of claim 1,wherein the solvent of step (b) is one or more selected from a groupconsisting of tetrahydrofuran, diethylether, dioxane, acetonitrile,acetone, methyl isobutyl ketone, ethylacetate, isopropyl acetate,n-butylacetate, dichloromethane, chloroform, 1,2-dichloroethane,benzene, toluene, xylene, dimethylacetamide and dimethylformamide. 7.The process of claim 1, wherein in step (b) the acid selected from agroup consisting of hydrochloric acid, sulfuric acid, formic acid,nitric acid, acetic acid, trifluoroacetic acid, p-toluenesulfonic acid,methanesulfonic acid and benzoic acid is used in an amount of 1.5equivalents with respect to the oxazolidine substituent-containingtaxane of formula
 4. 8. The process of claim 1, wherein in step (b) thebenzoyl chloride is used in an amount of 1 to 1.2 equivalents withrespect to the oxazolidine substituent-containing taxane of formula 4.9. The process of claim 1, wherein the base of step (b) is one or moreselected from a group consisting of sodium bicarbonate, potassiumbicarbonate, sodium carbonate, potassium carbonate, sodium hydroxide,potassium hydroxide and lithium hydroxide.
 10. The process of claim 1,wherein in step (c) aqueous ammonia or ammonia-organic solvent solutionhaving a concentration of 5 to 40% is used in an amount of 1 to 5equivalents with respect to the compound of formula
 5. 11. The processof claim 1, wherein in step (c) the salt of ammonia with a weak acidselected from a group consisting of formic acid, acetic acid andpropionic acid is used in an amount of 1 to 5 equivalents with respectto the compound of formula
 5. 12. A taxane derivative represented by thefollowing formula 3: ##STR27## in which R₁ represents trihaloacetyl. 13.The compound of claim 12, wherein R₁ is trichloroacetyl.
 14. A processfor preparing the taxane derivative of formula 3 as defined in claim 12characterized in that (d) a 10-deacetyl-baccatin III represented by thefollowing formula 8; ##STR28## is reacted with trihaloacetyl halide in asolvent in the presence of a base to provide a10-deacetyl-7-trihaloacetyl-baccatin III represented by the followingformula 9: ##STR29## in which R₁ represents trihaloacetyl, then (e) thecompound of formula 9 thus obtained is reacted with acetyl halide in asolvent in the presence of a base.
 15. The process of claim 14, whereinin step (d) the trihaloacetyl halide selected from a group consisting oftrichloroacetyl chloride, trichloroacetyl bromide, tribromoacetylchloride, tribromoacetyl bromide, trifluoroacetyl chloride,trifluoroacetyl bromide, triiodoacetyl chloride and triiodoacetylbromide is used in an amount of 1 to 1.5 equivalents with respect to thecompound of formula
 8. 16. The process of claim 14 or 15, wherein thetrihaloacetyl halide is trichloroacetyl chloride.
 17. The process ofclaim 14, wherein in step (e) the acetyl halide is used in an amount of1 to 8 equivalents with respect to the compound of formula
 9. 18. Theprocess of claim 14 or 17, wherein the acetyl halide is acetyl bromide.19. The process of claim 14, wherein in step (d) or (e) the solvent isone or more selected from a group consisting of tetrahydrofuran,diisopropylether, methyl t-butylether, dioxane, methyl isobutyl ketone,acetonitrile, ethylacetate, isopropylacetate, n-butylacetate, pentane,hexane, heptane, dichloromethane, chloroform, 1,2-dichloroethane,benzene, toluene, xylene, dimethylacetamide, dimethylformamide andpyridine.
 20. The process of claim 14, wherein in step (d) or (e) thebase is one or more selected from a group consisting of pyridine,triethylamine, imidazole, DBU, diisopropylethylamine, potassiumt-butoxide, sodium ethoxide, n-butyllithium, phenyllithium, lithiumdiisopropylamide, sodium hydride and lithium bistrimethylsilylamide. 21.An oxazolidine derivative represented by the following formula 2a or itssalt: ##STR30## in which X is as defined in claim 1, andR representshydrogen or C₁ -C₃ alkyl.
 22. The compound of claim 21, wherein it isthe salt form represented by the following formula 2b: ##STR31## inwhich X is as defined in claim 1,R₅, R₆ and R₇ independently of oneanother represent C₁ -C₄ alkyl or phenylalkyl, or R₅ and R₆ together canform a cyclic chain having 4 to 7 ring atoms.
 23. The compound of claim21 or 22, wherein X is chloro.
 24. A process for preparing theoxazolidine derivative as defined in claim 21 or its salt characterizedin that a (2R,3S)-phenylisoserine derivative represented by thefollowing formula 10: ##STR32## in which R is as defined in claim 21, isreacted with a compound represented by the following formula 11:##STR33## in which X is as defined in claim 1, in a solvent in thepresence of an acid catalyst.
 25. The process of claim 24, wherein thesolvent is one or more selected from a group consisting oftetrahydrofuran, diisopropylether, methyl t-butylether, dioxane,acetonitrile, ethylacetate, isopropylacetate, n-butylacetate,dichloromethane, chloroform, 1,2-dichloroethane, benzene, toluene,xylene, dimethylacetamide and dimethylformamide.
 26. The process ofclaim 24, wherein the acid catalyst is one or more selected from a groupconsisting of p-toluenesulfonic acid, methanesulfonic acid,trifluoromethylsulfonic acid, pyridinium p-toluenesulfonate andamberlite IR-120.
 27. The process of claim 24, wherein the compound offormula 2a in which R is alkyl is further hydrolyzed in a solvent in thepresence of a water-soluble base to obtain the compound of formula 2a inwhich R is hydrogen.
 28. The process of claim 27, wherein the solvent isa mixture of water-miscible organic solvent and water in a ratio of 10:1to 100:1 by volume.
 29. The process of claim 28, wherein thewater-miscible organic solvent is one or more selected from a groupconsisting of methanol, ethanol, isopropyl alcohol, tetrahydrofuran,dimethylformamide, dimethylacetamide, acetone and acetonitrile.
 30. Theprocess of claim 27, wherein the water-soluble base selected from agroup consisting of lithium hydroxide, sodium hydroxide and potassiumhydroxide is used in an amount of 1 to 2 equivalents with respect to thecompound of formula 2a in which R is C₁ -C₃ alkyl.